Impulse generator



April 9 E. LABIN ETAL 2,418,128

INVENTORS [MIL E 1. 7 BIN EVE PT MflN UE L 05 TLUND sygxifxgjafhATTURNKY' Patented Apr. 1, 1947 UNITED STATES PATENT OFFICE IMPULSEGENERATOR Emile Labin, New York, N. Y., and Evert Manuel Ostlund,Montclair, N. J., assignors to Federal Telephone and Radio Corporation,Newark, N. J a corporation of Delaware Application February 13, 1943,Serial No. 475,738

(Cl. 1719'7l 3 Claims.

This invention relates to radio impulse systems and more particularly tothe generation of impulses for use in pulse modulation systems and otherelectrical systems where high power impulses are desirable.

High voltage surges have been generated here tofore by use of circuitscontaining spark gaps known generally as lightning generators." Thesecircuits are used chiefl for high voltage testing of electricalinsulators and power equipment. These lightning generator circuits.while capable of producing high peak voltage and current with steep wavefronts are not suitable for such uses as pulse modulation. This islargely due to the fact that the impulses generated by such circuitshave a duration which is relatively long, the pulse repetition rate isvery low and the wave front is usually excessively steep.

It is one of the objects of this invention to provide a system utilizingspark gaps for generating and shaping impulses applicable for pulsemodulation of radio frequenc transmitters with-- out the aforementionedand other objections of the spark gap circuits heretofore proposed.

Another object of this invention is to provide a simplified impulsegenerator capable of furnishing short duration. high voltage. high peakpower impulses at high repetition rates and of selected shapeparticularly useful for pulse modulatin purposes in radio detectionsystems such as that disclosed in the copcnding application of E. LabinSerial No. 473.310. filed January 23, 1943. The Labin system is arrangedto operate directly on raw alternating current and therefore avoids thenecessity of much of the weighty apparatus required to furnish directcurrent heretofore believed necessary for radio detection systems. Thepresent systn which may derive its power from an alternating currentsource is particularly useful as a pulse modulator for theaforementioned Labin radio detection system.

The above and other objects of this invention will become more apparentupon consideration of the following detailed description to be read inconnection with the accompanying drawings. in which:

Figs. 1 and 2 are schematic wiring dia rams of two forms of impulrcgenerators in accordance with this invention;

Fig. 3 is a graphical illustration of the operation of the two forms.shown in Fig 1 and 2, and: Fig. 4 is an enlar ed view showing thegeneral shape of a high voltage impulse produced in accordance with thisinvention.

Referring to Fig. 1 of the drawings, the gener- 2 ator therein showncomprises three condensers C1, C2 and C3 adapted to be charged inparallel through isolating resistors R1, R2, R3, R4 and R5 from anunfiltered half-wave rectifier comprising rectifier tubes Ill and H. Thecondensers C1 and C2 are connected in series relation by a spark gap Si,the condensers C2 and C3 are connected in v series relation by a secondspark gap S2 and the output side of the condenser C3 is connected to athird spark gap S3. By this arrangement, the three. condensers arecharged in parallel arrangement an d dischar ed in series through a loadcircult comprising a shaping inductance L1 and a load resistance Re. Thevoltage peak output is approximately three times the voltage to whichthe ondensers C1, C2 and C3 are charged. While three condensers andthree spark gaps are shown in this circuit, it will be understood thatany other number may be used. giving a peak output voltage equal to acorresponding multiple of the condenser charging voltage. It will alsobe understood that any suitable type of rectifying means may besubstituted for the tubes l0 and I I.

The power supply for the generator may comprise any available source ofalternating current applied to the input of the circuit at the terminalsl5. The power supply is applied through a switch is to a filamenttransformer l1 whereby the filaments l8 and IQ of the tubes l0 and IIcan be preliminarily heated before the generator is used for generationof impulses. A switch 20 is provided to control application of the powerto an autotransformer 2| which is connected to a transformer 23 adaptedto provide a potential upon the plates 24 and 25 of the tubes l0 and H.The auto-transformer provides an adjustment for varying the platepotential. The spark gaps S1. S2 and S3 are made adjustable and may beinterconnected as indicated by the broken lines 21 and associated withthe control 28 of the autotransformer 2 i.

The spark gap Si in the absence of a source of synchronizing pulses fortriggering the generator may be adjusted closer than the gaps S2 and $3.This will enable the system to be triggered by the potential of thepositive half-cycle of the alternating current applied at the terminalsl5. Thus, as the potential 01 the applied power rises from zero to thepositive crest the gap S1 will be caused to break down at a selectedpotential and thereby provide a conducting path. With the first gapconducting, current flows in circuit loops czRcRisi and CiRiSi so as toproduce breakdown voltage across the second gap S2. Due to the extremclyhigh overvoltage impressed across this age breakdown characteristics.

gap, it becomes conductive in a very short time. 01 the order of a smallfraction of a microsecond. Breakdown of the two gaps S1 and S2 connectsC1. C2 and C3 in series with the output gap 8: which fires to connectthe condensers in series with the load circuit LIRO. Complete dischargeof the series bank oi condensers takes place through the load circuitand the series parallel combination of isolating resistors in parallel.The charging circuit is non-conductive in the reverse direction as wellas isolated by series resistance during this portion of the cycle.

The voltage across the gaps drops rapidly to zero in a time determinedby the time constant of the load resistance and the series condensercombination. Deionisation oi. the gaps occurs at this zero voltage pointrestoring their high volt- Deionisation of the gaps is also eilfectedrapidly at this point because of the flat face shape of the spark gapelectrodes. This flat face shape provides maximum surface from which amultiplicity of paths of electrons are capable of building-up rapidlyonce the gap arcs across, and since the electrode faces are flat, theseare paths may vary in location continuously back andforth thereacross.This prevents the electrodes developing high temperature spots whichwould tend to prolong the are by establishment of a. concentratedelectron flow. In other words, by properly shaping the electrodes asillustrated schematically in Fig. l a maximum surface is provided forre-combination of the ions and since points of concentration are avoidedblow-out action of the active ions is in eiiect provided. Consequently,a rapid recovcry of the initial high breakdown voltage of the spark gapsis eifected by the quick extinction oi. the are by this spark gapconstruction.

The rate of discharge of the series combination or three condensers, isdetermined by their capacitance, the inductance of shaping coil L1 andthe resistance R6 of the load circuit. The rate of rise of the dischargepulse is determined by the value of inductance Ll which may beconveniently made of such a value as to give desired output pulserise-to-decay time ratics. The limitation on L1 is then that it may notbe larger than the critical value of the inductance for the dampeddischarge circuit. It is evident that with small values of L1, the.total condenser voltage will be obtained across the output. As L1 isincreased, the output voltage decreases until for the critical dampedcase, 74% of the condenser voltage is obtained in the output.

The behavior of the circuit on discharge is determined by the followingequations. The general expression for the load current for the dampedcase is:

1 i= sinh t amperes (l) E=peak series condenser voltage; R=loadresistance (Rs) L=inductance (L1); and

where 111 cI a n 1 5 inntanh -seconds (2) and the maximum current isexpressed by:

imwfic i g amperea (3) where L0 For the limiting case of criticaldamping, the current is:

= tr" amperes (4) which gives for the maximum value i,,,,..=0.74%"ampercs (5) In Fig/3, a sinusoidal wave 40 is shown in the form of analternating current applied as the source of power to the terminals I5.The rectifier tubes Ill and I I provide, in response to the alternatingcurrent, voltage pulsations 4| for successive positive half portions ofthe alternating current wave. These pulsations are applied to thecondenser-resistance circuits Rlcl, R2CzR4 and RsCoRs. The wave 44illustrates the charging of the condensers in response to the voltagepulsations 4|. The condensers charge up to a point 45 at which the sparkgap S1 is adjusted to breakdown thereby causing a sharp voltage drop 46.The discharge of the condensers in series, represented by the voltagedrop 46 produces a sharp voltage rise 41 (Fig. 4) across the outputcircuit the build-up and duration of which is controlled by theinductance Ll, the resistance Re and the series capacitance C. By properchoice of the values 01' these discharge parameters, the voltage impulse48 is shaped so as to be applicable as a source of high voltage energyfor pulse modulating purposes. The rapid deionisation of the spark gapsrendering them non-conductive after the drop 40 (Fig. 3) of the voltageacross them below their extinction potential, insures the desired zerocurrent interval between pulses and accordingly a high peak to averageoutput current ratio. A typical output impulse 48 is shown in Fig. 4,having in this case a six microsecond duration. This is, of course, avery small fraction of the total time interval of 16,666% microsecondsof a single cycle of 60-cycle alternating current such as commonly used.

Referring back to Fig. 3, it will be noted that the voltage pulsation 4|continues after the voltage drop 46, the amplitude of the pulsation, ofcourse, decreasing thereafter. This portion of the pulsation 4| startsto re-charge the condensers C1, C2 and C3 and the re-charge is indicatedby the level 49. Since the negative portions or the alternating current40 are not rectifled by the tubes l0 and II, the voltage level 49continues until the next positive portion of the alternating currentwave occurs. The condensers are then charged as before until the sparkgaps breakdown. The pulse output, when the gap firing is dependent uponthe rising charge upon the condensers as described above, isseml-synchro nous, that is, the firing time may vary slightly one way Oranother point. Where true synchronized pulse output is desired, thegenerator is provided with a source of synchronizing pulses arranged tocause breakdown of the spark gaps at a selected point along a cycle ofthe alternating current wave. For exfrom a, selected discharge ternallysynchronized operation, the gaps are so adjusted as to be non-conductivefor the peak voltage normally attained by the condensers in response toa cycle of alternating current. At the proper time during the charging,as when the condensers have attained a desired voltage, a synchronizingvoltage pulse having a steep wavefront and of short duration is appliedover an input connection 54 across the resistor R4. This pulse has sucha polarity, in this case negative, as to increase the voltage across thespark gap S1 beyond the breakdown value'thereof thereby causing it tofire. The series discharge of the condensers which follow is ashereinbefore described. Such synchronizing voltage impulse may beapplied, of course, at other points in the circuit so as to produce therequired voltage rise across a spark gap.

In Fig. 2 a circuit similar to that of Fig. 1 is shown wherein therectifier is a full wave rectifier comprising tubes 50 and 5| having theanodes 52 and 53 thereof connected across the sec-. The cathodes of.

ondary of a transformer 55. the tubes '50 and 5| are interconnecting andthe output thereof is applied to one side of the resistor-condensercircuits RiCi, R2C2R4 and RaCsRs. The other sides of these circuits areconnected by a connection 56 to ground and to a mid-tap on the secondaryof the transformer 55.

This circuit (Fig. 2) operates in a manner similar to that of Fig. lwith the exception that voltage pulsations 42 are provided for thenegative portions of the alternating current wave 40 (Fig. 3). Thus, byusing a full wave rectifier pulses are generated at twice the repetitionrate of the generator of Fig. 1.

From the foregoing description, it will be clear that the generators ofthis invention provide for synchronous or semi-synchronous or randompulse output, as the case may be, and at a high repetition rate. Byproper selection of circuit constants, such as the transformer 28 andthe spacing ofthe spark gaps as controlled by the connection 21, arandom or limited random pulse output may be obtained. The pulseproduced has a duration which is maintained short, and the pulse isprovided with a wave-front the steepness of which is controlled. Inother words, the discharge of the generator is highly damped and givesstrictly a single output pulse. The rapid deionisation of the spark gapsinsures hiflh damping discharge operation, and by proper selection ofthe time constants of the circuit the charging time of the condensers ismaintained shorter than the repetition period of the pulses.

While we have shown and described two specific forms of apparatus toillustrate the principles of the invention, it will be understood thatthey are shown by way of illustration only and not as limiting theinvention as set forth in the objects thereof and in the appendedclaims.

What is claimed is:

1. An electrical impulse generator comprising a circuit having aplurality of condensers connected in parallel, means including aplurality of spark gaps each disposed in series connection with respectto said condensers, means to supply energy to said condensers, means tocontrol breakdown of said gaps to effect discharge of the energy storedin said condensers in series additive relation through said gaps toproduce a large discharge impulse, an output load resistance, aninductance coil in series between said spark gaps and said output loadresistance to control the rise-to-decay time ratio of the producedimpulse.

2. The generator defined in claim 1 wherein the gap breakdown controlmeans comprises REFERENCES CITED The following references are of recordin the fileof this-patentz UNITED STATES PATENTS Number Name DateLusignan Apr. 9, 1935

